with electrics connected, cold engine, ign on, plugs should power up for approx 8 secs
cold engine, wait for glow light to extinguish, start engine and let idle, plugs should power up for approx 30 secs.

but to be honest, unless you know the history of plugs, may pay to change them anyway
originally you stated all was fine once started...so..

injector coding...if done in right sequence, did you get numbers from injectors themselves, or the sticker...if it was the sticker, you may find injectors have been changed/swapped about in past...would double check those codes on the injector themselves.
would blank that egr off and fit a quality filter and make sure all is bled properly..plus clear any stored codes

once that is all done, then if you still have problems, see what code/s are flagging up

Update: Cleaned EGR valve yesterday, seems like it's stopped black smoking, but it hasn't had any effect on the starting / glow plug problem. Blanking plate has been ordered and i'm going to read up on cleaning the inlet manifold as I didn't have time to do that after work yesterday.

Some other things I hadn't mentioned:

The problem started when it was snowing and the car was left un-started for quite a few days, could this have damaged something? At first I could jump it off from another car's battery but that stopped working (I had the battery tested and I was told it was fine). When I turn the engine off the car really judders to a holt and the car shakes quite badly, Finally, I noticed that the fuel access hole seems quite open, there is only a piece of metal hanging down, but quite a substantial gap around it, which doesn't seem right (pic attached).

Once the car has been started, it usually runs ok for the rest of the day, but sometimes it takes longer to fire when turning the key. Depressing the accelerator while cranking improves this, it feels like there is a lack of fuel getting through..